Enzymic oxidation of fatty acids on a porous bed solid support

ABSTRACT

This invention relates to a process for enzymic oxidation of a fatty acid to produce an oxidation product in the presence of water and oxygen characterised in that the fatty acid, water and enzyme for the oxidation are substantially uniformly distributed throughout a porous bed of solid support material in the substantial absence of a continous liquid phase, oxygen is passed through the porous bed and the oxidation product is recovered from the porous bed. The invention is of particular value for production of methyl ketones from saturated fatty acids. Apparatus for carrying out the process is also novel and comprises a sealable vessel (1) that can contain a porous bed 4 of solid support material, a gas inlet (12) for supplying oxygenating gas to the reactor, a gas outlet (13), passage means (7) defining a plurality of upwardly extending passages that can open along their length into the bed and that communicate with the inlet (12) and that are preferably spaced 5 to 20cm (preferably 5 to 15cm) apart, and means for forcing oxygenating gas through the inlet and along and out from the passages and into the bed.

This invention relates to a process for aerobic enzymic reactions foroxidation of fatty substrates that can be carried out in relatively highconcentrations and can permit the recovery of high yields of desiredproduct.

BACKGROUND OF THE INVENTION

Enzymic hydrolyses and oxidation processes occur naturally both in solidand liquid media, and generally give a range of products. The crudemixture may be unpleasant, but individual components may be desirable.Therefore, it is known to perform the processes artificially foroxidation of naturally occurring materials.

Commercial aerobic enzymic processes are normally carried out insolution. This has to be relatively dilute to allow oxygen to bedistributed easily throughout the mixture, as otherwise aeration becomesdifficult. Thus, the biotransformation solution must be a dilutesolution and the yields resulting from such a dilute solution arerelatively low.

Solid state processes are also known. The solid phase can be thesubstrate that is to be converted (e.g., malting of barley grains) orcan be a support for the aqueous medium in which the oxidation occurs.In Process Biochemistry, July 1966, J. Meyrath describes production ofamylolytic enzymes by growth of various molds which produce such enzymeson bran and on vermiculite to obtain a yield of the amylase enzyme. Inaddition, Oriol et al., in the Journal of Fermentation Technology,volume 66, number 1, pages 57 to 62, describe how Aspergillus nigerfungus was grown on a solid support of sugar cane bagasse.

The solid state process described by Meyrath is for the production ofthe enzyme itself. However, it is also known to use the enzymes formedto obtain the biotransformation products that they produce. Anothersolid state reaction is discussed in Advances in Applied Microbiology,volume 28, pages 201 to 237, for the production of citric acid on inertmaterials. Sawdust and sugar-free sugar cane bagasse were used as thegrowth support, and this was impregnated with sugar solution. The inertmaterials were inoculated with mold culture, and citric acid wasproduced. Other growth supports mentioned are rice hulls and wheat bran.

It is known that natural atmospheric oxidation of fats and oils producesa crude, unpleasant product (e.g., rancidity), but that some componentsof the product can be useful.

In Phytochemistry (1984) volume 23, number 12, pages 2847 to 2849,Kinderlerer describes natural oxidation of coconut due to the action ofmolds natural to the coconut as giving a series of aliphatic methylketones and secondary alcohols. In Phytochemistry (1987) volume 26,number 5, pages 1417 to 1420, she described their production in a liquidmedium, following growth of Aspergillus ruber and A.repens fungi withcoconut oil as the sole carbon source. However, low yields wereobtained.

It is also known to use milk and milk compounds as a growth medium forbiotransformation reactions. For example, ketones have been produced bybiotransformation processes on milk and milk compounds. In U.S. Pat. No.3,100,153, a process is described wherein pasteurized homogenized milkcontaining milk fat is fermented under submerged aerobic conditionsusing Penicillium roqueforti to produce ketones, and in U.S. Pat. No.3,720,520, ketones giving a blue cheese flavor are produced by growingPenicillium roqueforti with aeration in an aqueous medium of sodiumcaseinate and fat.

Such blue cheese flavors are also described in pages 285-287 of volume40 (1975) of the Journal of Food Science, where R. Jolly and F. V.Kosikowski report studies of biotransformation of coconut fat and butterfat with whey powder, carried out in an aqueous medium, again usingPenicillium roqueforti.

U.S. Pat. No. 4,769,243 describes the preparation of green aromacompounds by reaction of at least one unsaturated fatty acid (which maybe produced in situ from fat and lipare) with enzymes from soy beans. Anaqueous solution of water and ground germinating soy beans is mixed withlinseed oil, and the mixture is allowed to react while stirring rapidly.Air or oxygen is supplied to the mixture throughout the reaction. Theresulting reaction is due to the enzymes present in the germinating soybean. This type of liquid process must be stirred rapidly to ensure thatthe mixture remains substantially homogeneous. In addition, it isdifficult to provide oxygen throughout the whole reaction mixture, andso liquid mixtures as described above must be relatively dilute andrequire some form of intermixing. If the amount of water is reduced, thefat tends to form a very viscous continuous phase between the particlesof ground bean, and this will prevent adequate oxidation.

It would be desirable to devise a process of enzymically oxidizing fatsand/or fatty acids and that can be performed easily and to give highyields of the desired products. It would also be desirable to providenovel apparatus suitable for such processes.

SUMMARY OF THE INVENTION

According to the present invention, there is now provided a process forenzymic oxidation of a fatty acid in the presence of water and oxygen toproduce an oxidation product characterized in that the fatty acid,water, and an enzyme for the oxidation are substantially uniformlydistributed throughout a porous bed of solid support material in thesubstantial absence of a continuous liquid phase; oxygen is passedthrough the porous bed, and the oxidation product is recovered from theporous bed. The oxidation product may undergo further reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

Suitable apparatus is described with reference to the accompanyingdrawings in which:

FIG. 1 shows a cross-sectional view of one apparatus.

FIG. 2 illustrates a cross-sectional view of a second apparatus.

FIG. 3 is a diagrammatic representation of another apparatus thatincludes recycle.

FIG. 4 shows a partially cut away perspective view of one arrangement ofrod passage means in the bed-containing portion of an apparatus.

FIG. 5 shows a cross-sectional view of a rod with removable core andsheath in position before insertion into a bed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The starting material is a fatty substrate which can be fatty acidalone, or mixed with fat or other ester, or fat alone (which ishydrolysed to acid prior to the oxidation process).

The fatty acids used by be saturated or unsaturated hydrocarbon fattyacids, or may be saturated or unsaturated hydroxy carboxylic acids.

Saturated fatty acids are oxidized in the beta position and willsubsequently undergo decarboxylation. Unsaturated fatty acids undergooxidative cleavage to give products having carbonyl groups at the pointof cleavage. These aldehydes or ketones may undergo subsequent reductionto the corresponding alcohols. Decarboxylation of the oxidation productsof saturated fatty acids is usually spontaneous due to thethermodynamically unstable nature of the oxidized compounds, but mayresult from a further enzymic reaction. Saturated, unsaturated, andhydroxylated fatty acids react to produce methyl ketones, aldehydes, andlactones, respectively.

Secondary alcohols can be produced as the final end product by reductionof product methyl ketones or primary alcohols from aldehydes: forexample, if a reductase enzyme is present.

Unsaturated aldehydes and alcohols can be produced from doubly orpolyunsaturated higher fatty acids such as linoleic or linolenic acid.Oxidative attack at one of the unsaturated groups breaks the double bondto produce, for instance, two fragments, each terminating with acarbonyl group. Particularly useful products of this type of reactionare hexenals and hexenols. For example, linseed oil contains doubly andtriply unsaturated C₁₈ glycerides which result in the formation ofunsaturated fragments, giving rise to hexenals and hexenols.

The invention is of particular value for the production of methylketones from saturated fatty acids.

A vital feature of the invention is that the oxidation is conducted on asupport that is porous throughout the reaction, since this permits easyaeration and contact of substantially all parts of the material withoxygen. To maintain this condition, the support cannot be oil-logged orin the form of an aqueous suspension or slurry, and so this is incontrast to U.S. Pat. No. 4,769,243, which uses an aqueous slurry.

Since the liquid phase is carried on the porous bed of solid support,its aeration is controlled by the porosity of the bed. The concentrationof the fatty acid in relation to the aqueous phase can be relativelyhigh, and so the yield of product from the process is maximized. Itwould be difficult to obtain satisfactory aeration of a bulk liquidphase of the same concentration because of its physical properties. Forinstance, the maximum yield obtainable by a conventional liquid phaseprocess would be around 15% or less, but in the invention it is possibleeasily to obtain yields of above 20%, and much higher if optimumconditions are selected.

The process has an additional advantage in that since it is abio-process, as long as the starting products are of natural origin andnot artificially produced, the products of the process can be classifiedas natural products.

The support is a porous matrix which may be fibrous (for instance anopen web of fibers), but is preferably particulate. The particles may benon-porous, but preferably they are a porous particulate substance sothat the surface area of the support is maximized. The bed can then beregarded as both macroporous (due to the interstices between particles)and microporous (due to porosity within particles). The particle size isgenerally from 0.001 to 5 millimeters, preferably from 0.01 to 2millimeters. Examples of suitable support materials are inert,non-carbohydrate materials that may be inorganic or organic (generallysynthetic polymeric) materials such as vermiculite and fibers or foamswhich may be made of plastic materials. Preferably the support is acarbohydrate, preferably a cellulose such as cellulose powder, riceflour, maize starch, wheat flour, woodpulp, or other carbohydrate porousmatrices fibers or other particulate materials.

In the invention there must be substantially no continuous aqueous orother liquid phase. When a particulate material is used, the supportpreferably remains substantially friable throughout most (and preferablyall) the process. Although some aggregation of particles is acceptable,the individual particles or aggregates of them should remainsubstantially separate from one another in order that porosity andaerobic conditions prevail substantially throughout the bed.

The enzymes may be pre-formed, or may be grown in situ. They must besuitable to induce the desired oxidative reaction of fatty acids.Pre-formed enzymes can be obtained from any suitable source, and shouldbe mixed substantially uniformly throughout the carbohydrate ornon-carbohydrate bed. The pre-formed enzyme may be obtained from a plantsource, or may be of microbial origin. Examples are ground germinatingsoy beans in aqueous suspension or enzymes isolated from them, oralcohol oxidase from Pichia pastoris, or alcohol dehydrogenase fromBaker's yeast.

Preferably, however, the enzymes are generated in situ, for example, bygrowth in the porous support of micro-organisms capable of producing thedesired enzymes for effecting the desired conversion. This is ofparticular value when the fatty acid is part of a fatty substrate duringthe process which comprises both a fat and a fatty acid derived from thefat. The micro-organism should be distributed uniformly throughout thebed, as otherwise a proportion of the fatty substrate will not beexposed to the aerobic biotransformation. This uniform distribution canbe achieved by for instance absorbing it into the particles while it iscarried in, for instance, an aqueous nutrient medium. It is particularlypreferred to use a filamentous micro-organism since then the growth ofthe micro-organism on the support results in the growth of filamentswhich carry the micro-organism to other parts of the bed. Themicro-organism can be any of the molds that grow naturally on the fattysubstrate or can be any other micro-organism capable of effecting theenzymic biotransformation of the fatty acid. When the micro-organism isbeing grown in situ, and especially when filamentous, the support ispreferably unstirred during the process. The porous bed itself may move(e.g., on a conveyor), but the relative movement within the bed shouldbe kept to a minimum.

An important feature is that when the enzyme is produced in situ, alimited source of carbohydrate for growth of the enzyme-producingmicro-organism should be present throughout the bed. This source may bea soluble carbohydrate such as dextrins, glucose, or variousmonosaccharides. It may be added to a non-carbohydrate support, butpreferably it is released from an otherwise insoluble carbohydratesupport that may, for instance, be contaminated with for instance 0.1 to5%, often about 1%, soluble carbohydrate before use. The preferredcarbohydrates listed above generally provide a suitable source as wellas being suitable supports.

It is generally desirable that the support material releases orotherwise provides part of the source of carbohydrate available to themicro-organism during at least most of the process, especially when thesupport is static and the micro-organism filamentous. Generally, thefatty substrate also serves as a source of carbon. However, in someinstances it can be desirable to include a small amount of solublesaccharide, such as sucrose or glucose, in the aqueous medium to promoteinitiation of the process. If soluble saccharide is included after theprocess has been initiated, the product yield is liable to drop,possibly because the micro-organism is metabolizing the solublesaccharide in preference to the fatty product and the insolublecarbohydrate.

A mold that will provide the desired enzyme is preferably mixed into thesupport as spores which germinate on the support to form mycelia.Alternatively, mycelia may be added to the support; but this isgenerally not preferred, as mycelia are much less robust than moldspores and are liable to be damaged during distribution throughout thesupport. Growth conditions should be carefully regulated during thegermination stage to obtain optimum results as the micro-organism isvery sensitive to adverse conditions during this stage.

The germination stage produces mycelia which generally takes from around1 to 6 days, and the optimum temperatures for this stage are ambient:for example, 15° to 30° C., preferably 18° to 25° C. In addition, thegermination stage is sensitive to acidity, and although it has beenfound that free fatty acids can be added to the support bed for enzymicoxidation, the smaller chain fatty acids have been found to tend toprevent or impair the rate of germination. Accordingly, germinationshould preferably be conducted in the substantial absence of fatty acidsof less than 8 carbon atoms. During this initiation phase, little or nooxidation of fatty acid takes place.

When germination is conducted in the presence of fatty acids,germination is followed by a lipolysis stage in which fat is hydrolysedto the fatty acid components.

The micro-organism then induces oxidation of fatty acid and this cancause a rise in temperature typically from ambient to between 35° to 50°C.

In order to limit excessive temperature rise, a cooling means may beused, and this is preferably a cooling coil and/or jacket. Thetemperature in the bed is generally below 50° C., preferably below 40°C., and most preferably below 35° C., throughout the reaction.

The fatty acid may all be pre-formed by any natural process (and thefatty substrate will then consist solely of fatty acid), especially whenpre-formed enzyme is used. When the fatty acid is pre-formed, preferablyit is added to the support bed gradually, as conversion occurs, becauseif the acid content in the bed is too high, the enzyme action may beaffected. This is a particular problem with short chain fatty acids, andthe problem can be minimized by using only fatty acids having a carbonchain length of ten or above. Preferably, some or all of the fatty acidis generated by hydrolysis in situ of a corresponding ester (for examplean ethyl or other alkyl ester, but preferably a glyceride) either by ahydrolyzing enzyme produced by micro-organism grown on the support, orwith the addition of some pre-formed lipase to effect the initialhydrolysis of fat or oil to fatty acid. In situ production of fatty acidis advantageous in that it gives a means of controlled release of fattyacid to the support bed and acts as a rate-limiting step i the process.The fatty substrate during the process will then contain both fatty acidand fat, but at the start of the process (i.e., before the lipolysis)may consist solely of fat.

Preferably the process comprises introducing the fatty acid as a fat(which may be a solid fat or an oil) and growing a micro-organism thatprovides both a lipase for hydrolysis of the fat and an oxidase foroxidation of the resultant fatty acid to ketone or aldehyde.

It is often preferred for some of the fatty acid to be produced from afat or oil in situ and some of the fatty acid to be added to thereaction mixture, since this can increase yield. The fatty substrate canthen comprise different types of fatty acids, and generally also fat.When the fatty acid is added to the support bed after the germinationstage, it is distributed into the support bed in order to produce asubstantially even distribution throughout the bed while aiming for theminimum disturbance of the bed. For instance, the fatty acid can beblended with separate porous support material, and this can then beblended with the bed containing the germinating microorganisms sincethis reduces the amount of stirring needed to obtain uniformdistribution.

A separate, first lipolysis stage may be provided in a separate reactorin which fat and lipase enzymes such as Candida lipase are reacted toproduce fatty acid which is subsequently added to the bed. Preferablythe subsequent addition of fatty acid to the bed is regulated.

Preferably, the fat used is coconut oil, but other suitable fats or oilsinclude castor oil, linseed oil, arachis oil, sunflower oil, maize oil,palm leaf oil, palm kernel oil, or any edible oil. The amount of fattysubstrate added as fatty acid and the amount added as fat can varyconsiderably. The source of fatty acid may comprise 100% fat or it maycomprise 100% pre-formed fatty acid. However, in the preferred processwhere the hydrolyzing enzyme is formed in situ, the ratio of fat tofatty acid is generally between 1:10 and 50:1, and preferably from 1:5to 10:1 by weight.

Particularly preferred fatty acids are those which occur naturally (asglycerides), and the preferred fatty acids are C₆₋₁₈ aliphaticcarboxylic acids which may be saturated or unsaturated and may have oneor more hydroxy groups.

In the case when the micro-organism is also the source of lipase andoxidase, and the fat used is, for example, coconut fat, the fatty acidpreferably has a carbon chain length less than C₁₆, and most preferablyless than C₁₄, because these lower carbon chain lengths appear to bepreferentially oxidized by the micro-organisms. However, using other fatsources and/or other micro-organisms, preferred fatty acids may differ.We hypothesize that the reason for this is that initial fat hydrolysisis an extracellular reaction resulting from the lipase enzymes which aresecreted by the micro-organism and act outside the cells to producefatty acids, and the resultant fatty acids must be capable of migrationinto the cells of the microorganism through the cell wall, for thesubsequent oxidation and decarboxylation, which is an intracellularreaction.

Suitable micro-organisms for producing enzyme or enzymes for hydrolyzingand oxidizing oils, fats, and fatty acids such as those discussed above,are molds from the Phycomycetes class, or the Ascomycetes or Fungiimperfecti classes. Preferably, those used are food grade molds such asPenicillium roqueforti, Penicillium cameberti, Aspergillus niger,Aspergillus oryzae, Rhizopus japonicus, or Rhizopus oryzae. Non-foodgrade molds are also suitable for the biotransformation, and examplesare Penicillium cyclopium and Eurotium herbariorum. They convenientlycan give methyl ketones.

Enzymes that are particularly suitable for converting linseed oil andsimilar polyunsaturated materials can be provided by any green leafplant. Grass or germinating soya beans are examples of preferredmaterials because they have a relatively high content of the enzyme.

Octenols and hexenols can be obtained from germinating soya beans withunsaturated fatty oils such as linseed oil. Castor oil and carotenoidsmay also be used as substrates in conjunction with grass, since thisvegetation provides suitable enzymes.

For preparation of the support bed, the fatty substrate (which may be asolid or an oil at ambient temperature) may be warmed to assist loadingby ensuring that the viscosity is low so that it will mix easily ontothe support. Preferably, it is mixed with the particulate materialbefore addition to the vessel in which fermentation is to occur. Themixing can be achieved using a mixing machine: for example, a Nauta orWinkworth mixer. Water or aqueous nutrient is also loaded onto thesupport in this way and may be added either prior to or following theaddition of the fatty substrate.

Alternatively, the fatty substrate and water or aqueous nutrient may bemixed to form an initial emulsion which is then loaded onto the support.It may be necessary to add a suitable emulsifier to maintain theemulsion until it is loaded onto the particulate material to ensure thatboth components are distributed substantially evenly throughout thesupport. Suitable emulsifiers are, for example, lecithin, casein, andquillaja. Artificially produced emulsifiers may also be effective, butare less preferable for production of food grade products.

When the enzyme is pre-formed, either fat or fatty acid can be loadedonto the support. However, when the enzyme is produced in situ by themicro-organism, it is important to select the optimum conditions forenzyme induction in order to maximize growth and bioconversion. Asexplained above, due to the sensitivity of the germination stage, it isgenerally essential that the acid present should be at a relatively lowconcentration, and in particular there should be substantially no freefatty acid present having a short carbon chain length. It has been foundthat while the short chain fatty acids, for example C₈, are damaging orretarding to the germination stage, a C₁₀ fatty acid may be present andthe germination can still take place.

When the germination stage is substantially complete, pre-formed fattyacids can be added directly to the support. These are generally C₆ -C₁₈,and preferably C₈ and/or C₁₀ fatty acids. Preferably the fatty acids areadded as a mixture of different carbon chain length fatty acids, such asoccurs naturally. The acids can be added as, for instance, alkali metalsalts.

The weight ratio of fatty substrate to water on the support should becarefully controlled for optimum yields, and is generally from 1:10 to7:10, preferably from 1:3 to 4:7; and most preferably around 2:3. Ifthere is too much fatty substrate, a fatty layer tends to separate outinto the bottom of the reaction vessel and the process loses efficiency.If the amount of water is too high, the yield is reduced and there is atendency for the bed to become saturated with water if the amount offluid is increased to compensate for the low yield.

When the enzyme is formed in situ and a fatty acid is added to thesupport bed prior to germination, the substrate is preferably added at aconcentration of from 2% to 30%, and most preferably the concentrationis from 5% to 10%, based on the total weight of loaded support bed.

Where the fatty acid is added to the support bed after germination of anenzyme-producing mold, it is again generally added at a concentration offrom 2% to 30%, and preferably 5% to 15%.

Concentrations of fatty acid below 2% can be used, but tend to beimpractical, while the upper limit of fatty substrate that can be usedis generally imposed by the possible loading of fatty substrate andwater onto the support material and the moisture requirement of themicro-organism for effective growth.

In choosing suitable support materials, it should be noted that themicro-organism can affect the capacity of the support for the aqueousphase. For example, it has been found that for a particular fattysubstrate-to- water ratio, the tendency for separation of the two phasesis increased following inoculation of micro-organism into the bed ofsupport material and its consequent rise in temperature.

In addition, the ratio of water to the support material and fattysubstrate to the support should be controlled. Although it is desirableto have a high content of liquid phase, the support bed should not befully saturated to the point where it will tend to form a continuousphase and reduce the passage of oxygen so that some parts of the supportbed will, at least in part, no longer be aerobic.

The ratio of water to the support material is generally from 1:10 to6:3, preferably from 1:3 to 5:3, and most preferably around 4:3.

It is naturally desirable that the load of fatty substrate and aqueousnutrient solution, or water, be as high as practicable withoutsaturating the support and destroying its porosity. Generally, the totalamount of fatty substrate and the aqueous nutrient solution or waterthat is absorbed into the support is from 15% to 99%, often 50% to 90%,by weight of the amount required to cause complete saturation asindicated by the formation of a substantially continuous phase. Forinstance, when as is often preferred, the support medium can absorbaround 2 to 3 parts solution and fatty substrate per part supportmaterial, it is often convenient to absorb into the particles 0.7 to 1.3(often around 1) part fatty substrate and 0.7 to 1.5 (often around 1.3)aqueous nutrient solution or water per part by weight support.

All the above ratios are optimum for a support bed of cellulose powder.However, they are also suitable for most supports, although some mayrequired slightly different amounts for optimum results.

Where the enzyme is produced in situ, an appropriate nutrient sourcemust be included: for instance, to ensure that all of the standardessential trace elements of biotransformation are available. Someessential elements will already be available as inevitable traceelements in, for example, the support material. Examples of essentialelements are iron, copper, manganese, molybdenum, and selenium. Thenutrient source is preferably included in the water that is loaded ontothe support, in order to promote growth of the enzyme-producingmicro-organism. Thus, the nutrient source and water are preferably anaqueous solution that provides adequate inorganic nutrients for thebiotransformation, but typically can be Czapek type medium (often at 1.5to 2.5, preferably about 2, times the normal concentration). Thus, theaqueous solution typically contains nutrients such as sodium, magnesium,potassium, zinc, copper, and iron; and generally contains 1.5 to 6(preferably 3 to 5) g/l sodium nitrate, 0.3 to 2 (preferably about 0.8to 1.2) g/l magnesium sulphate heptahydrate, 0.5 to 2 (preferably about0.8 to 1.2) g/l potassium chloride, and trace quantities of ferroussulphate, zinc sulphate, and copper sulphate.

It is naturally important that there should be no extraneous poisoningcontamination present in the porous bed. It is a recognized problem inmany microbiological processes that some contaminants may interfere withor stop desired microbiological processes at certain concentrations, andso the materials and conditions used in the invention should not be suchas to produce contamination by toxic concentrations of potentially toxicmaterials. For instance, if a toxic impurity might be leached fromapparatus used in the process by contact with an acidic medium, contactbetween fatty acid and that apparatus should be avoided.

The aqueous medium may be buffered to a pH at which the process proceedsmost efficiently. Although the process can be performed at pH values inthe rage of, for instance, 4 to 8, best results are generally achievedwhen the pH is in the range of 6 to 7.5, and most preferably when it isbuffered with phosphate to a pH of about 6.8. However, as discussedabove, fatty acids such as C₁₀ may be added to the support bed after theinitial germination, and in these cases the pH will be lower.

The desired product formed during the process may escape from the poroussupport bed as it is produced and thus can be collected from gasescaping from the bed during the process. However, this may beunnecessary since the amount of product evolved as vapor may be verylow, and the bulk of the product may be trapped in or on the support andthe bed that it forms; and the desired products can be recovered fromthe bed at the end of the process. The trapping can be either byabsorption into any porous particles in the support, or by dissolutionor adsorption with the organic materials (especially mycelium) therein,or both. It is particularly surprising and convenient that the processcan be substantially run to completion before recovering the productsfrom the support bed.

Recovery of the product of the bioconversion can be achieved bydistillation, preferably by mixing the contents of the bed in water,distilling the mixture, and recovery of the distillate.

The collected product is a mixture containing natural components, andthis mixture itself can be very useful. For example, the product fromcoconut oil is a mixture containing natural ketones having excellentblue cheese flavor properties. However, the individual components of theproduct mixture are particularly useful; and these can be obtained fromthe mixture by a normal procedure of fractional distillation, preferablyunder vacuum.

The process may be carried out in a biotransformation vessel whichcontains the support bed and that includes means for aerating the bed soas to ensure sufficient oxygen is available for the required oxidativeprocess at substantially all positions in the bed.

The process generally may be carried out in a semicontinuous orcontinuous process (for example on a conveyor), but is preferablyconducted batch-wise.

It is generally desirable for the bed to be static and unstirred, sincestirring the support bed may tend to help form a continuous liquid phasewhich will reduce porosity and may prevent air permeation through thebed, in addition to causing damage to mycelia.

Generally, the bed is less than 2 meters (preferably less than 1 meter)deep since increasing the depth of the bed increases the pressure in thebed and the risk that the friable mixture will compress and tend towardsa continuous phase. Generally the bed is greater than 0.1, andpreferably greater than 0.5, meters deep.

Air or oxygen should be passed through the support bed at a ratesufficient to stimulate rapid biotransformation, but insufficient tofluidize the bed. A satisfactory rate is 0.25 to 5 liters of air perminute, and more preferably 1 to 3 liters of air per minute, per 1000kilograms of the loaded support bed.

It is often preferred that the concentration of oxygen in the gas supplyshould be below the oxygen content of air since better results areobtained using a high volume of a dilute gas than with a low volume of aconcentrated gas, because it is easier to control the actual amount ofoxygen delivered to each part of the bed. Preferably the gas contains 1%to 10% oxygen, often 2% to 5%, with the balance being inert gas such asnitrogen.

One way of achieving this dilute oxygen is by recycling the oxygenatinggas within a loop consisting of a bio-reactor or including one or morereactors, and adding sufficient oxygen or air to the recycle to maintainthe desired concentration. The recycle will have a high content ofcarbon dioxide; and we have surprisingly found that this does notinterfere with the process, and that the carbon dioxide is an inert gasin this process. Accordingly, a preferred gas is 1% to 10% (often 1% to5%) oxygen, above 50% CO₂, and 0% to 49% nitrogen. The amount of CO₂ isoften 70% to 90%.

When recycling, it is necessary to prevent escape of too much water fromits desired location and, in particular, it is necessary to ensure waterdoes not condense in the recycle and cause localized waterlogging of abed.

The biotransformation vessel is preferably supplied with a gas inlet andwith an outlet. Both connections may be fitted with filter units,scrubbers, or valves in order to prevent microbial contamination.

One way to ensure substantially even distribution throughout thebiotransformation vessel is to force gas up through the bed from adistribution manifold across the base of the bed.

If the vessel is a substantially airtight cylindrical or other vesselhaving a removable lid, then it can be convenient to have a flexible gassupply duct leading down through the lid and support bed to below themanifold. A gas outlet is generally provided towards the top of thevessel so that waste or product gases can be removed. Simple apparatusof this type can be a substantially cylindrical vessel with a capacityof approximately 50 to 450 liters, and most preferably from 180 to 340liters.

Instead of passing the oxygenating gas upwards, it is often preferred topass it laterally through the bed from a series of spaced-apart gassupply passages that each supplies gas into the bed over substantiallyits entire length. The extent of permeation will depend on the materialof which the bed is formed. It can be determined by simple experiment,but is generally not more than about 10 cm and is often 5 to 8 cm. Thus,the separation between two passages should be not more than about 20 cm,often not more than 10 to 16 cm. This technique has the advantage ofproviding more uniform oxygen concentration than by upflow from amanifold.

Preferred apparatus for use in the invention is novel and can be usedfor a variety of fermentations conducted using a porous bed (such ascitric acid production) or for the fermentations of the invention. Itcomprises a sealable bio-reactor that can contain the porous bed; a gasinlet for supplying oxygenating gas to the reactor; a gas outlet;passage means defining a plurality of passages that can open along theirlength into the bed and that communicate with the inlet and that arepreferably spaced 5 to 20 cm (preferably 5 to 15 cm) apart; and meansfor forcing oxygenating gas through the inlet and along and out from thepassages and into the bed.

The invention includes both the empty vessel and the vessel filled withthe bed. The passage means can be horizontal and spaced apart bothhorizontally and vertically, but preferably they are substantiallyvertical.

The spacing of the passage means relative to one another is dependentupon the porosity of the bed, and this is largely dependent upon theparticle size of the support material forming the bed. The passage meansare spaced so that gases can permeate to substantially all areas of thebed.

One suitable passage means comprises a plurality of rod shaped passagesthat provide outer annular gas passages. For instance, each passage canbe defined by a permeable tube, allowing gas to escape along its length,preferably at a substantially uniform rate along the length. However,the annular passages can also be formed lo by forcing tubes through thebed so as to create an annular layer around each tube that is of lowerdensity than the rest of the bed. For instance, each tube can beinserted while covered by a sheath, and the sheath can then be removed.To prevent blockage of the tube during insertion, the tube can have aremovable core which can be removed after insertion so that air can thenflow down the center of the tube, out at the lower end, and up throughthe low density annulus around the rod. Additionally, low densityannulus may be formed by shrinkage of the bed away from the tube duringuse.

Another suitable passage means comprises a substantially self-supportingopen network that preferably extends up through the bed and thatcommunicates at its base with the inlet. Gas is forced from the inletinto the base of the network and flows up the network and out into thebed from most or all of the surface area of the network. Preferably thenetworks are substantially parallel to one another and not more thanabout 10 to 15 or 20 cm apart. Generally, two networks define the sidesof an annular bed.

In order to maintain the desired humidity in the bed, there ispreferably a trough in the base of the bed in which water can collectwhen in use, and a wicking layer of fabric extends up from this troughover part or all of some or all of the networks, so as to promoteevaporation of water into the gas surrounding the networks and into thebed. The evaporation also helps maintain a uniform cool temperaturethroughout the bed.

The process of the invention can also be conducted using flat trayscarrying a shallow layer of support or a drum method as described inAdvances in Applied Microbiology, volume 28 (1982), "Solid SubstrateFermentations," by K. E. Aidoo, R. Hendry and B. J. Wood, page 209.

The apparatus in FIG. 1 comprises an air-tight vessel (1) having asealable closure (2) and an inner body (3) defined by cylindrical walls(32) extending upwards through the vessel, but which does not extendfully to the top of the vessel. This defines an annular space (4) insidethe vessel. A fan (5) is positioned at the top of the inner body (3) forcirculating the gases in the vessel down through air ways (6) cut in thewalls (32) of the inner body around its base and leading into theannular chamber (4). A bed of porous support material on which thefermentation takes place is positioned in the annular space (4). Gasinlet (12) is used to provide oxygen and any other required gases to theapparatus, and outlet (13) is used for removal of exhaust gases.

A gas permeable open network (7) is provided around the inner and outerwalls of the annular space. The network is provided by an open mesh ofsubstantially rigid, self-supporting material which extends from thebottom of the vessel upwards towards the top of the vessel.

As heat is produced during biofermentation, water evaporates off the bedand is liable to condense on the sides of the vessel and run down intothe base of the vessel. Therefore, in order to prevent water-logging ofthe bottom portion of the support bed, an annular raised platform (9) isprovided in the annular space and above air ways (6) so that a trough orchamber (10) is provided beneath the bed for collection of condensedwater and circulation of gases. In addition, in order to provideredistribution of this condensed water to the support bed a second layer(11) is provided next to the mesh layer (7) inside the vessel around theannular walls and comprises a wicking material. The wicking layer (11)is positioned so that its lower edge reaches substantially to the bottomof the vessel into chamber (10) so that it is in contact with anycollected condensed water. The wicking action of the layer then drawsthe condensed water back up the side of the vessel where it can bereabsorbed by the porous support bed either by evaporation or byconduction.

It has been found that, using the bed of porous support materialdescribed in the examples, adequate oxygen distribution is achieved forup to approximately 8 cm from the edge of the bed. Therefore, in orderto achieve effective oxygen distribution throughout the porous bed, thedistance between the gas permeable layers (7) should be no greater than16 cm. This distance may vary with porosity of the support bed.

The apparatus may be arranged in other ways: for example, a series ofconcentric annuli may be provided for containing the support bed, eachannulus separated from the next by a gas permeable layer (optionallywith a wicking layer).

FIG. 2 illustrates a similar apparatus except that the wicking layer isomitted and there are cooling coils (14). Cooling water enters thecooling coils via inlet (15) and leaves the cooling coils via exit (16).Evaporated water from the support bed condenses on these cooling coilsand, because of their position above the annular space (4) forcontaining the porous bed of support material, condensed water fallsback into the support bed and is redistributed. FIG. 2 also shows fluiddrive (17) for the hydraulic powering of the fan (5).

The reactor may be any shape, but for convenience is preferablycylindrical. The inner body may be integral with the vessel or may be aseparate body positioned in the bottom of the vessel. The apparatus canbe made of any suitable material which will not interferedisadvantageously with the reaction inside the vessel, but preferably isprepared from a material which can be autoclaved for sterilization: forexample, plastics such as polycarbonate plastic, polypropylene, or othersuitable materials. The fan (5) may be powered by any suitable means,but is preferably powered by pneumatic or hydraulic means as thisminimizes safety hazards.

The gas permeable network may be formed from any material which will notinhibit the reaction in the reactor, but is preferably made fromautoclavable material. Generally, the gas permeable network is formedof, for example, plastics materials in which the diameter of the networkstrands is preferably sufficient to make the mesh self-supporting, butsmall enough to leave substantial spacing within the layer. For example,the strands may be from 0.5 to 5 mm thick, preferably from 1 to 2 mmthick, and preferably at least 50% of both the transverse andlongitudinal cross-section areas of the layer comprises open pores inthe network.

The wicking layer may be formed from any material which will provide awicking action to redistribute the water: for example, an open wavecloth such as linen.

FIG. 3 is a diagrammatic representation of an alternative reactorsystem. The reactor vessel (20) is a closed, substantially air-tightcontainer having at its base a chamber (29) connected to the main bodyof the reactor by an air distribution plate (21). The reactor is alsoprovided with cooling coils (22), an exhaust gas outlet at the top ofthe reactor, and a gas inlet (30) at the base of the reactor. Theapparatus illustrated is a closed-loop circulation apparatus. Pump (23)draws gas out of gas outlet (24) and through condenser (25). Theescaping gas can carry heat out of the bed. As a result, gas is drawnthrough gas inlet (30) and through air distribution plate (21). Insteadof relying on vacuum to draw gas into the reactor, less preferably gascan be pumped into the vessel at a higher pressure.

The supply of available oxygen can be controlled by its addition to thecircuit. This addition may be made either by sucking in oxygen into thenegative pressure side, or by pumping oxygen in on the positive pressureside of the pump.

Gas can be recirculated through the apparatus so that the gases drawninto the apparatus are drawn from the pump via the condenser (26) andreservoir (27) which will collect any condensed water. Monitors may beintroduced into the circuit to detect the exact oxygen concentration.

The initial gas supply may enter the system via flow meter (28).

FIG. 4 represents a partially cut away perspective view of the portionof the apparatus that contains a cylindrical bed of support materialhaving an arrangement of rod passage means for distributing gasesthroughout the bed. Gases are supplied from an upper manifold (notshown) to the top of each of hollow tube (33) and down the length of thetube. Gases then exit hollow tubes (33) through exit holes (34) at thebottom of each tube and pass up the length of the tube in an annularzone around each tube and that is substantially free of particulatematerial. Additional holes (34) can be distributed along the length ofeach tube, if required, to provide a more uniform gas supply along thelength of each tube. The gas migrates laterally from around the tubesand into the porous bed. The spacing apart of hollow tubes (33) is suchthat gases can permeate substantially all areas of the bed. This isdependent upon the porosity of the bed and particle size of the supportmaterial. Exhaust gas leaves the support bed and exits from theapparatus via a gas outlet.

FIG. 5 represents an enlarged cross-sectional view of the lower portionof a tube with removable core and removable sheath in position, inpreparation for insertion into a bed. Tube (33) is provided with exitholes (34) at its end and along its length. A removable close-fittingsheath (35) is provided around hollow tube (33) and a removableclose-fitting core (36) is provided inside hollow tube (33). Thisarrangement is inserted into a bed of porous support material and, oncein place, the core (36) and sheath (35) are removed, leaving hollow tube(33) free of support bed material and with an empty annulus around thetube. Gases can then travel freely down the inside of the tube end andleave via exit holes to enter the empty annulus around the tube fortravel along the outside of the tube and permeation into the bed.

EXAMPLE 1

45 kilograms of coconut oil were dispersed onto 45 kilograms ofcellulose powder and mixed in a Nauta mixer. 60 kilograms of Czapek typemedium were mixed into the cellulose powder and coconut oil mixtureuntil an even texture was obtained. The Czapek type medium comprisedsodium nitrate 4 g/l, magnesium sulphate 1 g/l, potassium chloride 1g/l, ferrous sulphate 0.02 g/l, trace elements: zinc sulphate 0.01 g/land copper sulphate 0.005 g/l, with 0.075 moles/1 potassium phosphatebuffer at pH 6.8.

The final mixture was discharged into a biotransformation vesselcomprising an open-top drum having an air-tight lid and a flexible airinlet running through the mixture and to a manifold in the base of thedrum and a gas outlet in the lid of the drum.

Aspergillus niger had been grown on malt Czapek agar at 25° C. Thespores were harvested after approximately 10 to 14 days. Between 10⁷ and10⁹ spores per kilogram of loaded support bed were used for inoculation.Air was pumped to the vessel at a rate of 3 liters of air per minute.

Germination of the spores occurred over the first 2 to 6 days, and thetemperature rose from ambient temperatures to a maximum of 50° C.Elevated temperature were maintained over the course of thefermentation, and as the fermentation came to an end after between 20and 35 days, the temperature in the vessel began to fall gradually.

The methyl ketone products were separated from the fermentation mixtureby water distillation. The density of the oily product was 0.82 grams/mland the product separated out from the water and could easily beremoved.

A portion of this product was fractionally distilled under vacuum usinga column of twelve theoretical plates in order to obtain the individualmethyl ketones. The total yield of methyl ketones was approximately 16%,based on the weight of coconut oil.

EXAMPLES 2 to 4

Example 1 was repeated, but with the addition of capric and/or caprylicacid into the reactor after germination of the mold. The quantities ofCzapek type medium, coconut oil, and oxygen were varied. The totalyields of mixed ketones produced are shown in Table 1. The poor yieldsobtained for examples 2 and 4 were thought to be due to the effects oftoxic trace elements.

                  TABLE 1                                                         ______________________________________                                        Example:      2         3         4                                           ______________________________________                                        Amount of     22.5 Kg   3.00 Kg   3.00 Kg                                     coconut oil                                                                   Amount of     50.0 Kg   6.65 Kg   4.55 Kg                                     Czapek medium                                                                 Amount of     37.5 Kg   6.65 Kg   4.55 Kg                                     Cellulose powder                                                              Amount of     15.0 Kg   2.70 Kg   --                                          capric acid                                                                   Amount of     --        --        0.90 Kg                                     caprylic acid                                                                 Total weight  125.0 Kg  19.00 Kg  13.00 Kg                                    Rate of flow  3 1min.sup.-1                                                                           1 1min.sup.-1                                                                           1 1min.sup.-1                               of air                                                                        Yield of       9%       25%       12%                                         mixed ketones                                                                 (based on weight of                                                           fatty substrates)                                                             Yield of nonan-2-one/                                                                       17%       46%       25%                                         heptan-2-one                                                                  (based on weight of                                                           capric/caprylic acid)                                                         ______________________________________                                    

EXAMPLE 5

3 kilograms of coconut oil were dispersed into 3.5 kilograms ofcellulose powder using a Winkworth mixer. 3.5 kilograms of Czapek typemedium were mixed into the cellulose powder and coconut oil mixtureuntil an even texture was obtained (Czapek type medium as before). Thefinal mixture was discharged into a biotransformation vessel as shown inFIG. 1.

Aspergillus niger produced as for Example 1 was inoculated into thebiotransformation vessel. Air was pumped to the vessel at a rate of 1liter of air per minute. Germination of the spores occurred over thefirst 4 days, at which time a separately prepared mixture was added. Theseparately prepared mixture comprised 1.5 kilograms of capric aciddispersed into 1.75 kilograms of cellulose powder, followed by 1.75kilograms of Czapek type medium. This separately prepared mixture wasadded directly to the fermenting mixture in the biotransformation vesseland mixed together by stirring. The biotransformation vessel wasresealed and the air flow continued at 1 liter of air per minute.Elevated temperatures were maintained over the course of thebiotransformation, and the mixture was sampled after a total of 18 daysfermentation.

The volatile products in the sample were isolated by water distillation,and the composition of the oily distillate was determined by gaschromatography.

After a total of 29 days fermentation, the process was stopped and afurther sample taken for distillation and analysis of the oilydistillate.

The coconut fat was found by analysis to contain 6.1% of capric acid,which gives 274.5 g of capric acid (as glycerides) in the coconut fatused for the experiment. In addition, 1500 g of capric acid was added togive a total of 1774.5 g of available capric acid in a total mass of 15kilograms. This amount of capric acid is theoretically capable ofconversion into 1466 g of nonanone. After 18 days fermentation, thetotal content of nonanone in the 15 kilograms of fermenting matrix wasfound to be 907 g, which was a 62% conversion; and after 29 daysfermentation the content of nonanone was 1209 g, which was an 82%conversion. This example illustrates the improved yield obtained bycarrying out the process in the novel apparatus of FIG. 1 and is due toimproved control of temperature and aeration throughout the bed.

What is claimed is:
 1. An improved process for enzymic oxidation of afatty acid in the presence of water and oxygen wherein the improvementcomprises substantially uniformly distributing the fatty acid, water andenzyme for the oxidation throughout a porous bed of solid supportmaterial in the substantial absence of a continuous liquid phase;passing oxygen through the bed without fluidizing or stirring the bed;and recovering the oxidation product from the bed.
 2. A processaccording to claim 1 wherein the support bed is a macroporous matrix ofmicroporous fibrous or particulate material.
 3. A process according toclaim 1 wherein the support material is a carbohydrate selected fromcellulose powder, rice flour, maize starch, wheat flour, woodpulp andcarbohydrate fibers.
 4. A process according to claim 1 wherein the fattyacid is a saturated fatty acid and the improvement further comprisesdecarboxylating the oxidation product to produce a methyl ketone.
 5. Aprocess according to claim 1 wherein some of the fatty acid distributedthrough the bed is in the form of a fat.
 6. A process according to claim1 wherein the enzyme for effecting oxidation is pre-formed from a plantor micro-organism source.